Adds following macros:
* MRB_FIXED_STATE_ATEXIT_STACK (not defined by default)
  * When defined enables fixed state atexit stack.
* MRB_FIXED_STATE_ATEXIT_STACK_SIZE (default value: 5)
  * This macro will be ignored when `MRB_FIXED_STATE_ATEXIT_STACK` isn't defined.
  * When `mrb_state_atexit` is called more than this value it will raise runtime error.

Change to always use locals->car->car.

Fix clang analizer warnings

Fix conflict doc since there is only `add_conflict` method.

* master:
  update add_confict document; ref #2391
  Update document by regenerating it.
  Add :doc task to run document generator.
  Add document for `add_conflict` and `add_conflicts`.

Update document and add :doc task.

This reverts commit 29f14e72.

Fix memory leaks in String

Refactor how to use RSTR_NOFREE_P macro

Add NOFREE macros

rename mrb_atexit to mrb_state_atexit to clarify a func will be called at the end of state, not process; ref #2211